Respiratory Chain

• Made up of 4 complexes and 2 mobile carriers
  • Mobile carriers are CoQ and Cyt c
  • Flavin, Quinone, and Heme groups carry electrons from NADH to O2
• Complexes I, II and III have (Fe-S)
  • Act as electron carriers and liberate energy and electrons move down chain
    • Complex I and IV pumps out 4 hydrogens
    • Complex III pumps out 2 hydrogens
    • Complex II doesn’t have any proton pumps

Inhibitors

• Rotenone – blocks between FP1 --> CoQ
• Antimycin A – blocks flow from Cyt b --> Cyt c1
• Cyanide – blocks Cyt(a+a3) --> O2
  • Only one that reacts directly with oxygen
  • Outcompetes oxygen's affinity for hemoglobin

Flavin-Linked Dehydrogenase

• Contain FAD or FMN (rarely)
• NADH Dehydrogenase (FMN) + 8 Fe.S
• Succinate dehydrogenase (FAD) + 8 Fe.S
• Dihydrolipoyl dehydrogenase (2FAD)
  • Oxidizes lipoic acid and reduces FAD

Fe-S Proteins

• Non-Heme Iron Protiens
  • Present in Fe++/Fe+++ forms
  • Associated with sulfur atoms of cystein residues in protein
• Range from sinple to complex
• Plays crucial role in a wide range of redox reactions by participating in one electron transfers

Coenzyme Q10

• UBiQuinone
• Functions as a fat-soluable molecule shuffing between the flavoproteins and the cytochrome system in the lipid phase of the mitochondrial membrane
  • Electrons from NADH+H+ from Complex I, FADH2 from Complex II get shuffled to CoQ for transport to Complex III.
• Not a protein but a lipid!
  • Only component in electron transport chain that isn’t attached to a protein
• CoQ10 is the most common form and is ubiquitous in cells

Cytochromes

• Electron-transporting proteins that contain a heme prosthetic group
• All contain iron-porphyrin heme prosthetic group
• Transport electrons from CoQ to molecular O2
• Five different cytochromes
  • CoQ --> Cyt b --> Cyt c1 --> Cyt c --> Cyt (a+a3) --> O2
    • Cyt b and c1 is known as Complex III
    • Cyt (a+a3) is known as Complex IV
• Cytochrome oxidase is the only cytochrome that reacts with O2, forming H2O
  • All other cytochromes in the chain cannot react with O2
  • Cytochrome oxidase is inhibited by cyanide (CN-), Azide (N3), and CO

Heme

• Iron molecule bound by 4 nitrogens in a porphyrin ring
• Functions as electron carrier by oxidizing and reducing iron reversibly

Electron Transport Chain

• Complex I – NADH-ubiquinone reductase
  • Component – NADH dehydrogenase
• Complex II – Succinate-ubiquinone reductase
  • Component – Succinate dehydrogenase
• Complex III – Ubiquinol-cytochrome c reductase
  • Component – Cytochromes b and cytochrome c
• Complex IV – Cytochrome oxidase
  • Component – Cytocrome (a+a3)
• Complex V – FoFi ATPase or ATP synthase
• Three H+ needed to catalyze the formation of ATP from ADP + Pi

Oxidative Phosphorylation

• Phosphorylation of ADP coupled to respiration
• Mechanism for aerobic energy recovery
• Takes place in the mitochondria
• Occurs at the 3 energy conserving sites in the respiratory chain
  • Protons pumped into intermembrane space
    • Complex I and IV pumps 4 protons
    • Complex III pumps 2 protons
    • ATP synthesis are coupled by transmembrane proton fluxes
    • 3 protons is needed per ATP
    • Extra proton is needed to power an antiporter to move a Pi into the mitochondrial matrix while simultaneously moving a OH- to the intermembrane space
      • “Half” ATP is created because the pumps are pumping protons and the “half” ATP comes from the average ATP yield from the number of protons pumped by Complex I, III, and IV

Chemical-Coupling Hypothesis

• Requires formation of high energy intermediate

Conformational Hypothesis

• Requires a conformationally activated form of an electron carrier

Chemiosmotic Hypothesis

• Proposes that the formation of a proton gradient across the inner mitochondrial membrane is the primary energy-conserving event
• Uncoupled oxidative phosphorylation:
  • If H+ are somehow carried back into matrix, stimulating respiration without generating ATP
  • Can happen when there’s an uncoupling protein
  • H+ returning to matrix independent of ATP Synthase
  • Breaks the overall gradient
    • Produces heat due to the energy shifting, but no ATP
    • Is a common thermogenic reaction, seen primarily in the hibernators

Regulation of Oxidative Phosphorylation

• Concentration of ADP influences the amount of ATP that can be formed
  • Exerts respiratory control
    • Electrons are transferred to O2 only if ADP is phosphorylated to ATP
  • Entry of ADP into mitochondria requires the exit of ATP
    • Coupled flow of ATP and ADP is mediated by facilitated exchange diffusion
    • Inhibited by atractyloside, leading to cessation of oxidative phosphorylation

Genetics and Disease

• Mitochondria are the only organelles outside of the nucleus with their own DNA